liboverlay: define log2f only for jb_mr1

We only need this defined for JB MR1 as other builds may have it
defined in math.h

Change-Id: I6fbf3338bad63baa9dba3c7e135853bdce909930
diff --git a/liboverlay/overlayMdp.cpp b/liboverlay/overlayMdp.cpp
index 71e7e5b..61bfb19 100644
--- a/liboverlay/overlayMdp.cpp
+++ b/liboverlay/overlayMdp.cpp
@@ -29,10 +29,12 @@
         return ((int)(f1*100) == (int)(f2*100)) ? true : false;
 }
 
-//Since this is unavailable on Android, defining it in terms of base 10
+#ifdef ANDROID_JELLYBEAN_MR1
+//Since this is unavailable on Android 4.2.2, defining it in terms of base 10
 static inline float log2f(const float& x) {
     return log(x) / log(2);
 }
+#endif
 
 namespace ovutils = overlay::utils;
 namespace overlay {